Method of making permanent magnet material powders

ABSTRACT

THE COERCIVE FORCE OF A COBALT-RARE EARTH INTERMETALLIC COMPOUND IS GREATLY ENHANCED BY CHEMICALLY TREATING THE COMPOUND IN FINELY-DIVIDED FORM SO AS TO SMOOTH THE SURFACES OF THE INDIVIDUAL PARTICLES.

United States Patent Oifice 3,558,372 METHOD OF MAKING PERMANENT MAGNETMATERIAL POWDERS Joseph J. Becker, Schenectady, N.Y., assignor toGeneral Electric Company, a corporation of New York No Drawing. FiledJan. 31, 1968, Ser. No. 701,840 Int. Cl. H01f 1/06 U.S. Cl. 148-101 8Claims ABSTRACT OF THE DISCLOSURE The coercive force of a cobalt-rareearth intermetallic compound is greatly enhanced by chemically treatingthe compound in finely-divided form so as to smooth the surfaces of theindividual particles.

The present invention relates generally to the art of making permanentmagnets and is more particularly concerned with new magnetic materialpowders having unique characteristics and with a novel method forproducing these powders.

It is generally recognized that the permanent magnet properties of bulkmagnetic materials having large magnetocrystalline anisotropies can beenhanced by reducing them to powders. It is also common knowledge thatsuch powders can be incorporated in bonding media to provide compositepermanent magnets having properties substantially superior to those ofthe bulk source materials. These advantages are, however, offset to asubstantial degree in some instances when the particle size reduction isaccomplished by grinding. Thus, a comparatively low value of coerciveforce can substantially diminish the advantages to be gained byconverting the bulk body to a powder and fabricating a compositefinished article from the powder.

In accordance with the present invention, the detrimental eiTects ofgrinding upon the magnetic characteristics of the cobalt-rare earthmaterials can be eliminated and the coercive force ofmechanically-reduced materials of this kind can be enhanced to asurprising extent. In essence, the method of this invention centers inthe key step of chemically smoothing the surfaces of finely-divided,ground, magnetic material. This step is carried out with an acid mixturein a contact period of from a few seconds to 30 minutes during whichtime the individual powder particles preferably are not substantiallyreduced in size but their sharp edges and points are rounded andsmoothed by acid attack.

This invention thus centers in the concept of subjecting these groundmaterials to a chemical treatment to eliminate the degrading effects ofthe grinding upon their magnetic properties. The invention is also basedupon my discoveries that such treatment can result in surprisingly largeincreases in coercive force of these cobalt-rare earth materials, andthat this result can be obtained without incurring a significant productyield penalty or other substantial dis advantage.

As a general proposition, the chemical treatment is carried outaccording to this invention by contacting cobalt-rare earth magneticmaterial with a suitable acid solution, the material in powder formpreferably being immersed in the acid solution for the required periodof contact time and then promptly removed and rinsed free from thatsolution in order to arrest the acid attack.

Time and temperature are interrelated factors in this method, I havefound, however, that practical operating ranges of these variables areso broad that the necessity for precise control of the method canreadily be avoided. Thus, except for threshold or marginal conditions oftime and temperature, the operating conditions or a combination of theseconditions is not critical to the success or failure of the method interms of the products obtained.

3,558,372 Patented Jan. 26, 1971 In accordance with my preference, theacid mixture will be at rooom temperature but it may be at anytemperature at which the acid mixture is a liquid. The period ofimmersion or contact may likewise be anywhere from one or two seconds to30 minutes, my present preference being about 30 seconds. Prolonged acidcontact may lead to significantly diminishing magnet material yields,particularly if the treating solution temperature is substantially aboveroom temperature. At the other end of the time scale it is themechanical or manipulative limitation which generally is the importantone. Acid solutions I prefer for this process act rapidly enough thatmomentary contact is sufficient to insure consistently good results andit remains only to device means enabling immersion and removal of thepowder materials at the rate desired in a batchwise or a continuouschemical treatment operation. In any event a second or two would appearto be a reasonable immersion contact period and five to 10 seconds wouldbe an easier goal to reach in production operations.

For experimental purposes, I have used cobalt-rare earth magnet materialof particle size in the range between 250 mesh and +325 mesh (U.S.standard screen sizes). Materials of this kind may, however, be treatedaccording to this invention with the foregoing results when theparticles are twice as large, but the maximum coercive force obtainableis lower because of the fact that particle size is generally inverselyproportional to coercive force. Much finer particles may likewise betreated in accordance with this method but at the cost of smallerproduct yields because of the relatively larger proportion of eachparticle dissolved in the acid attack.

This invention method has been found in actual practice to beparticularly beneficial in actual practice in the treatment ofcobalt-base permanent magnet materials including Co Y, Co Sm and Co M(cerium-rich misch metal). The results obtained by applying the methodto materials of this kind are summarized in Table I.

TABLE I Coercive Initial force of coercive treated Particle force, AcidTime in material, size 1 oerstcds 2 solutions 4 seconds oersteds -250,+325 105 Acid A 10 1, 840 250, +325 20 2, 720 -250, +325 30 3, 340 -250,+325 7 s05 -5 30 4, 720 4 -20 10 6,400 250, +325 30 5, 250 -250, +325 304, 870 -250, +325 30 4, e -250, +325 2, 490 Acid 13.... 30 5, 520 -250,+325 1, 600 do 30 5, 230 -250, +325 6 1,290 do 30 4, 800 325 5 8, 100Acid A 20 8, 450 325 6, 430 do' 20 8, 300 -325 t 5, 200 d 20 s, 220 325870 30 2,940 325 870 .do.. 60 3, 370

1 U.S. standard screen sizes except where otherwise specified. 2Coercive force measured in field Hn=2 1,000 oersteds except whereotherwise stated.

3 Chemical polishing reagents A and O as given on page 328 of Smithell'sMetals Reference Book (Plenum Press, 1907) of composition as follows:

Acid A: Three parts nitric acid One part sulfuric acid One partorthophosphoric acid Five parts glacial acetic acid One part nitric acidOne part sulfuric acid One part orthophosphoric acid Five parts glacialacetic acid 200 grams chromic oxide grams nitric acid 10 grams sodiumsulfate Water to one liter volume. 4 Microns.

5 Hm=30,000 oersteds. 5 Hm=l8,000 oersteds. 1 M=Cerium-rich misch metal.

Acid B:

Acid 0:

applying this method to cobalt-rare earth permanent magnet materials canbe explained on the basis that the chemically-treated powders of thesematerials are smooth and free from sites for the nucleation of domainsof reverse magnetization. The improvement in the coercive forcecharacteristic of powders of such materials treated in accordance withthis method is such as to give support to this theory and there does notappear to be any alternative explanation for this remarkable change inthis key property of these matrials. Moreover, there does not appear tobe any chemical change produced in the magnet materials by the acidmixture treatment of this method, the only detectable difierences beingthe rounding of sharp edges and points and the great increase incoercive force of the powder particles.

The following illustrative, but not limiting example of an operationembodying this invention which I have carried out is oifered by way offurther describing the present novel method to those skilled in the art:

EXAMPLE Co Y was prepared by arc-melting cobalt and yttrium togetherunder an argon atmosphere and then casting the melt to form an ingot. Apiece of the resulting very brittle ingot weighing about one half gramwas ground with mortar and pestle and the resulting powder was screenedand the fraction passing a 250 mesh screen and return on a 325 meshscreen was selected for test. A part of this fine powder fraction wasintroduced intoa body of molten parafiin wax and the wax was cooled inan aligning magnetic field of about 21,000 oersteds until it wassolidified. The coercive force of this powder sample was measured as=being105 oersteds. The remainder of the -250 +325 mesh powder fractionwas placed in a dish with ml. of a mixture of 3 parts nitric acid, onepart sulfuric acid, one part orthophosphoric acid and five parts glacialacetic acid. The acid mixture was at C., and the powder was permitted toremain in the acid mixture for seconds whereupon the powder was removedfrom the acid mixture and rinsed with water and with acetone andpermitted to dry in air. This sample was then mounted inparaffin andtested as described above with the result that the coercive force wasfound to be 3340 oersteds, as stated in the third entry of Table I.

Whatever percentages or proportions are stated in this specification,results are made to the volume basis rather than weight basis.

Although the present invention has been described in connection withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

-What I claim as new and desire to secure by Letters Patent of theUnited States is:

1. In the method of producing a colbalt-rare earth intermetalliccompound denoted by Co R where R is a rare earth metal in a form inwhich it has superior coercive force including the steps of forming abulk body of said compound and grinding the bulk body to a powder, thecombination of the step of contacting the resulting fine-particlematerial with a chemical polishing reagent which is an acid solution andthereby substantially increasing the coercive force of the fine-particlematerial by smoothing the surfaces of the individual powder particles.

2. The method of claim 1 in which the bulk body is reduced to a powderby a grinding operation and in which the particle size of the finalpowder product is substantially the same as the particle size of thefineparticle material prior to contact with acid solution.

3. The method of claim 1 in which the acid solution consists essentiallyof three parts of nitric acid, one part of sulfuric acid, one part oforthophosphoric acid and five parts of glacial acetic acid.

4. The method of claim 1 in which the compound is Co Y.

5. The method of claim 1 in which the compound is Co Sm.

6. The method of claim 1 in which the compound is Co M wherein M iscerium-rich misch metal.

References Cited UNITED STATES PATENTS 1,878,589 9/1932 Marris 148-104X1,932,639 10/1933 Roseby 148104 1,998,840 4/1935 Legg etval. 1481003,421,889 l/1969 Ostertag et al. -170 3,424,578 1/ 1969 Strnat et a1.75-213 3,463,678 8/1969 Becker 148105 3,501,358 3/1970 Becker 148-105XL. DEWAYNE RUTLEDGE, Primary Examiner G. -F. WHITE, Assistant ExaminerU.S. C1. X.R.

